Support the MJML file contents being streamed in #72
Description
The plug-in is great but it only supports the MJML file already being present on disc so they they can be processed.
This means that if I have a gulp pipeline where the MJML file contents have been streamed in then it will not support that.
Would it be possible to add support for reading the MJML contents from the stream? I am happy to open a PR if this is something this project would be willing to support.